1. Field of the Invention
The present invention relates to a diaphragm-type carburetor, and in particular, to an improvement in a diaphragm-type carburetor including a constant-pressure fuel chamber having an outlet bore communicating with a lower end of a fuel nozzle through a fuel jet and a check valve, a fuel pump incorporated in a fuel passage which communicates between an inlet bore in the constant-pressure fuel chamber and a fuel tank for providing fuel for the constant-pressure fuel chamber in response to a pulsation pressure in a pulsation pressure generating source, and a fuel introduction control valve for controlling the introduction of the fuel into the constant-pressure fuel chamber by opening or closing the inlet bore in the constant-pressure fuel chamber. The fuel introduction control valve is provided with a cylindrical valve seat member mounted on an upper wall of the constant-pressure fuel chamber and having the inlet bore in its upper end, and a valve member lifted and lowered within the valve seat member to open and close the inlet bore.
2. Description of the Related Art
A diaphragm-type carburetor is already known, as disclosed, for example, in Japanese Patent Application Laid-Open No. 1-151758.
In such a carburetor, fuel delivered to a constant-pressure fuel chamber by operation of a fuel pump is often subjected to pressure pulsation received from the fuel pump, heat or vibration received from an engine or the like to produce fuel vapor. The fuel vapor introduced into the constant-pressure fuel chamber is ejected from a fuel nozzle along with the fuel, because the constant-pressure fuel chamber has no air vent (to enable the operating attitude of the engine in all directions). In the prior art, however, the fuel vapor may stagnate in the constant-pressure chamber depending on the operational attitude of the engine. When the operational attitude of the engine is changed, a large amount of stagnating fuel vapor may be ejected at one time from the fuel nozzle to extremely reduce the fuel-air ratio of the fuel-air mixture, thereby causing misoperation of the engine.